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hydromagnesite. The dypingite-like structure appears to have a smaller cell volume than the unnamed mineral and the 5H2O version of dypingite, as all peaks are shifted to smaller d-spacings (Fig. 3E). Hereafter, excluding hydromagnesite, [Mg5(CO3)4(OH)2 XH2O] minerals where X = 11, 8, 6, 5 or less are collectively referred to as dypingite-type mineral phases. The full-width at half-maximum values (FWHM) of the dypingite-type minerals are wider than hydromagnesite, suggesting greater long-range disorder in the dypingite-type minerals. Hydromagnesite shows similar FWHM throughout, even though the peaks have different relative intensities, as monitored by the peak at 9.6 degrees. The static experiment precipitates show nesquehonite-rich hydromagnesite bearing assemblages. There is no evidence of a shift in the abundance of nesquehonite relative to hydromagnesite with time, or the appearance of dypingite-type phases. Author's personal copy Phase transitions in the system MgO–CO2–H2O 5 Fig. 2. Experimental results for: (A) electrical conductance measurements (agitated environment symbol: , static environment symbol: ). AAS analyses of [Mg 2+ ]tot/(M) concentrations (static environment symbol: , agitated environment symbol: ); (B) pH measurements, (agitated environment symbol: , static environment symbol: ). Agitated environment solid phase samples are: [AS0], [A20], [A50], [A80], [A 120] and [A 240]. Static environment samples are: [S 60], [S 120], [S 180], and [S 240]. 4.3. FT-Raman All FT-Raman spectra contained significant noise, particularly those acquired from the agitated experiment. This relates to the acknowledged difficulty in analysing disordered magnesium hydrate and hydroxyl carbonates (e.g., White, 1974; Frost et al., 2009), compounded by the rapid synthesis under our experimental conditions. The [AS0] spectra (Fig. 4A) show a high intensity band at 1117 cm 1 , consistent with the v1 internal mode of [CO 2 3 ] for hydromagnesite (e.g., Edwards et al., 2005) and a range of low intensity bands assigned to hydromagnesite. In the 2500– 3800 cm 1 (H2O–OH) region, [AS0] shows an asymmetric band at ca 3660 cm 1 . The spectra of [AS0] samples collected after 5, 8, and 19 h of CO2 sparging are identical, suggesting that hydromagnesite precipitated directly from the parent solution.
The [A20] and [A50] spectra are very similar, showing a high intensity band at 1098 cm 1 , with a shoulder at 1117 cm 1 , plus a variety of low intensity bands assigned to nesquehonite and or hydromagnesite (e.g., Fig. 4B). Hence, results of the FT-Raman analysis support XRD evidence for a transition from hydromagnesite to nesquehonite formation. The spectra show broad bands centred at ca 1480 and 1780 cm 1 . Bands at similar frequencies have been assigned to the anti-symmetric stretching of [CO 2 3 ] and the water bending mode for nesquehonite (Hales et al., 2008). In the H2O–OH region, the asymmetric feature between ca 3000–3500 cm 1 contains poorly resolved overlapping bands at ca 3440, 3310 and 3156 cm 1 , the disposition of which are broadly compatible with synthetic [Mg(HCO3,OH) 2H2O] (e.g., Hales et al., 2008). A subsample of [A20] was analysed periodically over five days Author's personal copy 6 L. Hopkinson et al. / Geochimica et Cosmochimica Acta 76 (2012) 1–13 Fig. 3. X-ray diffraction pattern of samples: (A) [AS 0]; (B) [A 20]; (C) [A 80]; (D) [A 120]; (E) [A 240]. The star marked on the stick diagram of [Mg5(CO3)4(OH)2 8H2O], is the main peak enabling discrimination of the mineral phase from dypingite and hydromagnesite. of drying. The 1098/1117 cm 1 intensity ratio was uniform, suggesting that the loss of nesquehonite at the expense of [Mg5(CO3) 4(OH) 2 XH2O] phases was solely solventmediated. The [A80] spectrum shows an elevated (1117/1098 cm 1 ) intensity ratio relative to [A20] and [A50](Fig. 4C), consistent with the presence of hydromagnesite generated at 25 °C, combined with nesquehonite dissolution, and coeval formation of dypingite-type phases. The [A80] and [A120] spectra show broad low intensity asymmetric scattering in the ca 1000–1090 cm 1 region which is multi-component in nature (Fig. 4D). The lower frequency region coincides with Mg– OH deformation vibrations in dypingite (Frost et al., 2009). Low intensity scattering in the 1070 cm 1 region is consistent with the [CO 2 3 ] symmetric stretching mode (Frost et al., 2008). In keeping with XRD analysis, samples
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